Since the fluidized catalytic cracking (FCC) process was first introduced in the 1940s, the FCC process spent catalyst has been stripped with steam in a stripping section that is part of the reactor vessel. The purpose of the spent catalyst stripper is to strip out the hydrocarbon vapors entrained with the spent catalyst from the reactor section of the FCC process with steam. Typically, the steam enters the dense phase stripping section of the reactor at the bottom of the stripper, but in some of the newer stripper designs steam is introduced at two elevations in the stripping section of the reactor vessel and is referred to as two-stage stripping. In some designs, spent catalyst from the dense phase stripper has been transferred to a vertical riser and lifted with a lift gas to a secondary stripper before being sent to the FCC regenerator. In all such processes the stripper operates as a dense bed stripper with an average bed density across the stripper of 25 to 35 pounds per cubic foot. In the typical spent catalyst stripping section of modern FCC systems, the steam is introduced into the dense bed stripper at a rate of about 1 to 2 pounds of steam per 1,000 pounds of catalyst circulated. This rate of stripping steam results in a volume of steam vapor that is about equal to the interstitial volume of hydrocarbon vapors between the catalyst particles. Therefore, the rate of stripping steam normally in use in FCC units acts as a displacement media for the hydrocarbon vapors and not as a true stripping media (i.e., there is no upward velocity of steam).
This lack of adequate stripping is consistent with the observation that the accepted weight percent hydrogen in coke numbers for a typical FCC systems is 7 weight % at about a 7 to 1 catalyst to oil ratio, while the observed weight percent hydrogen in coke for a fluidized solids process operating at a 4 to 1 catalyst to oil ratio is only about 3.5 weight % and the observed weight percent hydrogen in coke in the ultra-short contact time catalytic cracking process (as hereinafter referred to) operating at 22.6 to 1 catalyst to oil ratio can be as high as about 21 weight %. An analysis of this data indicates that in the typical stripper design with 1 to 2 pounds of stripping steam per 1,000 pounds of catalyst circulation the stripper efficiency is very poor. These data and observations indicate that, depending on the operation, about 20 to 50% of the total coke burned in the regenerator is from hydrocarbon vapors entrained into the regenerator with the circulating catalyst. In fact, one can make the argument that the entrainment rate of hydrocarbon vapors with the spent catalyst circulated into the regenerator is very similar to the rate of "inerts" (products such as CO, CO.sub.2, H.sub.2, H.sub.2 O, SO.sub.x and the like resulting from the combustion of coke on spent catalyst) entrained with the regenerated catalyst into the reactor system, or about 1 to 2 pounds of inerts per 1,000 pounds of catalyst circulated.
Because the prior art spent catalyst stripper is a dense bed system and the unit hydraulics require it to operate in the 25--25 pounds per cubic foot density range so that the unit will circulate properly, it is practically impossible to make this system into a true stripper as long as it is part of the reactor vessel. If enough stripping steam were added to the stripping section to achieve good countercurrent steam to catalyst flow for stripping, the entrainment of spent catalyst back into the reactor vessel will increase and the unit will be limited on circulation capabilities as the density decreases. The re-entrainment of spent catalyst into the reactor system could result in undesirable reactions and products.
With the increased use of catalytic cracking, as well as the treating and upgrading of residual oil feedstocks and with the advent of ultra-short contact time catalytic cracking as described in my U.S. Pat. No. 4,985,136, issued Jan. 15, 1991, entitled "ULTRA-SHORT CONTACT TIME FLUIDIZED CATALYTIC CRACKING PROCESS"; the fluidized process described in my U.S. Pat. No. 4,859,315, issued Aug. 22, 1989, entitled "LIQUID-SOLID SEPARATION PROCESS AND APPARATUS"; and my U.S. Pat. No. 4,263,128, issued Apr. 21, 1981, and entitled "UPGRADING PETROLEUM AND RESIDUAL FRACTIONS THEREOF", the need for improvement in circulating fluidized solids stripper design has become apparent to me. All of the above-identified patents are incorporated herein by reference in their entireties.
I have determined that operating such fluidized catalyst or fluidized solid systems on heavy residual oil requires a better stripping design in order to reduce the hydrocarbon carryover into the regenerator, which will reduce the regenerator temperature and reduce the need for catalyst cooling. Further, improved stripping will also reduce the SO.sub.x emissions from the regenerator. Still further, reducing the amount of catalyst cooling in the FCC process will reduce the coke yield. Also, the ultra-short contact time catalytic cracking process, known in the industry as the "Milli-Second Catalytic Cracking Process" or "MSCC Process" increases the catalyst to oil ratio by a factor of 2.5 to 3. For example, if one were to use the normal stripper design criteria for a spent catalyst stripper, the stripping vessel would be bigger in diameter and longer, and there would be a 2.5 to 3 fold increase in the stripping steam rate.
Therefore, a primary object of the present invention is to greatly reduce, by use of an improved dilute phase elongated riser stripping section fluidized with a lift vapor such as water and/or steam (hereinafter referred to as stripping medium or stripping media), the amount of hydrocarbons entrained with the spent catalyst or other solid into the regeneration system. More specifically, there is a significant reduction in the amount of hydrocarbons and/or coke containing sulfur compounds which otherwise passes into the regeneration system, while at the same time providing catalyst cooling. This will reduce the FCC regenerator temperature and increase the catalyst to oil ratio to give a more selective reaction.
In one embodiment of the present invention, the use of regenerator catalyst/solids coolers can be eliminated by use of this unique process wherein water or water mixed with steam would be used as the lift media. The vaporization of the water will cool the spent catalyst/solid as well as the regenerated catalyst/solid so that the regenerator catalyst/solid coolers are not necessary.
Yet, another object of the present invention is to reduce the SO.sub.x (sulfur oxides) emissions from the regenerator by converting more of the sulfur compounds on the spent catalyst to H.sub.2 S by increasing the partial pressure of the water vapor in the stripper above that in conventional FCC strippers. This is more effective when the spent catalyst (or other solid) temperature is increased above 1000.degree. F. by recycle of hot regenerated solid or catalyst. It is commonly accepted that the sulfur in the coke and hydrocarbons that are burned in the regenerator will combine with a metal oxide to produce a metal sulphate that will be reduced and liberated as H.sub.2 S in the reactor in the presence of water vapor. The existing stripper limitations allow more material containing sulfur to enter the regenerator and reduce the amount of steam that can be used for stripping, which limits the partial pressure in the stripping section to push this reaction to completion. The present invention removes these limitations, with a resultant decrease in SO.sub.x in the regenerator flue gas emitted to the atmosphere because in the present invention there is used a dilute phase stripper whose effluent is maintained separate from the effluents of both the reactor and the regenerator.
Another object of the present invention is to separate the reactor and regenerator hydraulics in such fluidized systems so that the spent catalyst/solids stripper no longer has to be part of the reactor vessel, and therefore, the elevation of the reactor vessel can be lowered.
A further object of the present invention is to enable the use in such fluidized systems of higher stripping steam rates, which would not be possible in a dense bed stripper, and, also to permit recovery of more liquid and gas products as a result of improved stripping.
An additional object of the present invention is to enable reduction of the energy requirements for such fluidized systems as a result of the recycle and heat exchange of the stripping vapors.
Still another object of the present invention is to enable reduction of the catalyst losses from the regenerator of such fluidized systems by removing fines (solids of undesirably small particle size) from the circulating spent solid before they enter the regenerator.